Friction - Class 8 : Notes

Force of Friction:
(i) Friction is the force which resists the relative motion of surfaces.
(ii) All types of surface like solid, liquid and air offer the force of friction.
(iii) Friction force always applies in the opposite direction of movement.
(iv) Friction is applied when two surfaces in contact move relative to each other.
Examples:
(a) A moving wheel on the ground stops after covering a certain distance because surface of ground offers opposite direction friction to the surface of wheel in motion.
(b) After applying breaks to a moving bike, the surface of brakes offers friction to the surface of moving wheel and the bicycle stops because of force of friction.
(c) Gently push a book on a table. It stops after moving for some distance because of friction between the surfaces of book and table.

Factors affecting Friction:
(i) Friction is caused by the irregularities on the two surfaces in contact.
(ii) Even those surfaces which appear very smooth but microscopic view show those irregularities. When these surfaces come into contact, the irregularities of surfaces get interlocked. We have to apply force to remove the interlocking, a surface produces friction when it slides or rolls over another surface. 

(iii) Friction force will increase with increase in weight. As weight increases irregularities between surfaces get interlocked tightly because of pressure, and the surface offers more resistance of friction.
Example: It is easier to dragging a mat when nobody is sitting on it than a person is sitting on it because of friction increases with increase in weight.
(iv) Rougher surface offers more force of friction.

Types of Friction:
1. Sliding Friction:
(i) This friction comes in to action when a surface slides over another surface and resist its motion is called sliding friction.
(ii) Example: sliding of book or a box over ground.

2. Rolling Friction:
(i) This friction comes into action when a surface rolls over another surface and resist its motion is called rolling friction.
(ii) Rolling friction is smaller than sliding friction. Due to this roll anything over the ground is easier than to slide.

Friction: A Necessary Evil:
Friction is a necessary evil because it has both harmful and beneficial effects.
Examples:
(i) We can walk easily because ground offers friction. In the less friction walking situation becomes almost impossible. Therefore, friction is necessary even a simple task like walking.
(ii) It is difficult task to move on a wet muddy track, or wet marble floor. This is because these surfaces offer very small friction to the surface of feet of a person.
(iii) Soles of shoes have grooves on them. Grooved soles give more friction to the ground which gives better grip when we walk. Shoes with worn out soles can be slippery.
(iv) The tyres of vehicles have treads for better grip over road. It provides friction to the surface of road. When treads are worn out, the tyres need to be replaced with new one.
(v) Friction can also produce heat. Vigorously rub your palms together for a few minutes.
(vi) Writing with pen is possible because paper provides friction to the tip of pen.
(vii)When a machine is operated, heat generated causes much wastage of energy.

Increase of friction:
(i) Friction is desirable in some situations
(ii) Friction is necessary for us, therefore friction is increased to accomplish Some of important task.
Examples:
(a) Treaded tyres of vehicle increase friction. Vehicle with treaded tyres provides mores friction which gives better grip with the road.
(b)  Soles of shoes are treaded to increase friction between sole and the ground.
(c)  Javelin thrower rubs some powder on his palm to get better grip on the javelin.
(d)  Kabaddi players rub their hands with soil for a better grip of their opponents.

Reduction of friction:
(i) In Some situations, friction is undesirable. We need to reduce it.
(ii) With help of lubricant we can reduce the friction. Lubricants make a thin layer over surface and facilitate smooth movement.
(iii) In the engines of vehicles, use of lubricants increases the efficiency as parts are not rubbed directly each other because of thin layer of lubricant.
(iv) In many machines, graphite powder, oil, grease are used as lubricant for reducing friction.
(v) Few drops of oil are poured on the hinges of a door for reducing friction, the door moves smoothly.
(vi) While playing carom, sprinkle fine powder over it to reduce friction.

Fluid Friction:
(i) Gases and liquids are generally known as fluid. Similar to solid, fluids also provide friction. The friction of fluid increases with the speed of object moving through it and speed of fluid.
(ii) Objects which are made to move in fluid are made streamlined shape to reduce friction offered by fluid. The shape of aeroplane, ship, boat, birds and fish are made streamlined shape. A shape which is pointed on both ends and wide at the middle is called streamlined shape which facilitates easier movement through fluids.

(iii) The frictional force offered by fluids is called drag.